RFID system with time slot interleaving

ABSTRACT

A method for interleaving time slots in a multi-antenna system for communication with RFID tags is disclosed. An example is shown for an eight antenna system. A first four antennas arranged side-by-side are sequentially energized to interrogate RFID transponders. A second set of four antennas arranged side-by-side, the first of which is adjacent to the last of the first set of antennas. A four-antenna sequence is performed for the first four antennas and a second four antenna sequence is performed for the second set of antennas. The first and second four antenna sequences are offset by only a marginal amount, sufficient to ensure that a transponder signal received four antennas away from an active antenna is not acknowledged because the receive window for the non-active antenna is delayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility application claims priority as a Continuation Applicationunder 35 U.S.C. § 120 from Non-provisional application Ser. No.14/559,038 filed on Dec. 3, 2014, now U.S. Pat. No. 9,633,238, which isa Continuation of Non-provisional application Ser. No. 13/464,542 filedon May 4, 2012, now U.S. Pat. No. 8,928,462, which claims the benefitunder 35 U.S.C. § 119(e) of Provisional Application Ser. No. 61/483,568filed on May 6, 2011, all applications being entitled RFID System withTime Slot Interleaving, and the entire disclosures of which areincorporated by reference herein.

FIELD OF INVENTION

The invention relates to the field of radio frequency identification ofvehicles.

BACKGROUND OF THE INVENTION

Automatic Vehicle Identification (AVI) systems implemented using RadioFrequency Identification (RFID) transponders with active transmittershave a tendency for cross-lane reads and writes to occur in multi-laneenvironments as compared to systems implemented using backscattertechnology. Even though RFID reader systems typically have adjustmentsfor reducing the transmitted power and receiver sensitivity, theseadjustments may not provide the ability to squelch the response of anactive tag in an adjacent lane, or even the response of an active tagtwo lanes over without degrading overall performance of the system.

BRIEF SUMMARY OF THE INVENTION

In an embodiment, there is disclosed a method of communicating with anactive RFID transponder in a system having a plurality transponderreader antennas arranged as a first set of adjacent reader antennas anda second set of adjacent reader antennas and two sets of time slots forsequentially transmitting to the first set of adjacent reader antennasduring a first multi-position time slot and to the second set ofadjacent reader antennas during a second multi-position time slot. Themethod includes the steps of: sequentially transmitting a firstinterrogation signal to each of the first set of adjacent readerantennas during the first multi-position time slot; opening a firstreceiving window a predetermined time after each of the firstinterrogation signals, the first receiving window allowing reception ofsignals only from the one of the first set of reader antennas that justtransmitted; sequentially transmitting a second interrogation signal toeach of the second set of adjacent reader antennas during the secondmulti-position time slot; opening a second receiving window apredetermined time after each of the second interrogation signals, thesecond receiving window allowing reception of signals only from the oneof the second set of reader antennas that just transmitted; and delayingthe sequential transmission of the second interrogation signal to thesecond set off reader antennas from the sequential transmission of thefirst interrogation signal to the first set of the reader antennas, suchthat the second receive windows of the second set of reader antennas arenot aligned in time with the first receive windows of the first set ofreader antennas.

In a further embodiment, the first set of reader antennas consists offour reader antennas and the second set of reader antennas consists offour antennas. In a further embodiment, the first set of reader antennasconsists of three antennas and the second set of reader antennasconsists of three antennas. In a further embodiment, the first set ofreader antennas consists of two reader antennas and the second set ofreader antennas consists of two antennas. In a further embodiment, thefirst set of reader antennas is larger or smaller in number than thesecond set of reader antennas. In a further embodiment the delay islonger than the first interrogation signal.

In a further embodiment, there is disclosed a method of interrogatingactive RFID transponders including the steps of: transmitting a firstinterrogation signal from a first antenna and thereafter opening a firstacquire window; transmitting a second interrogation signal from a secondantenna a predetermined delayed time after the start of the firstinterrogation signal and thereafter opening a second acquire window;wherein the second interrogation signal is transmitted substantiallyduring the first acquire window. In a further embodiment, the first andsecond antennas are close enough to an RFID transponder that thetransponder can receive the first and the second interrogation signals.In a further embodiment, the first and second antennas are separated byadditional antennas and wherein the additional antennas transmitinterrogation signals at different times than the first and secondinterrogation signals.

In a further embodiment, there is disclosed a system for communicatingwith an active RFID transponder. The system includes: a transponderreader: a first set of adjacent reader antennas; and a second set ofadjacent reader antennas. The reader is adapted for sequentiallytransmitting to the first set of adjacent reader antennas during a firstmulti-position time slot and to the second set of adjacent readerantennas during a second multi-position time slot such that: a firstinterrogation signal is sequentially transmitted to each of the firstset of adjacent reader antennas during the first multi-position timeslot; a first receiving window is opened a predetermined time after eachof the first interrogation signals, the first receiving window allowingreception of signals only from the one of the first set of readerantennas that just transmitted; a second interrogation signal istransmitted to each of the second set of adjacent reader antennas duringthe second multi-position time slot; second receiving window is opened apredetermined time after each of the second interrogation signals, thesecond receiving window allowing reception of signals only from the oneof the second set of reader antennas that just transmitted; thesequential transmission of the second interrogation signal to the secondset reader antennas is delayed from the sequential transmission of thefirst interrogation signal to said first set of the reader antennas,such that the second receive windows of the second set of readerantennas are not aligned in time with the first receive windows of thefirst set of reader antennas.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary reader system for a four lane tollplaza;

FIG. 2 is a diagram of an exemplary reader system for a four lane tollplaza;

FIG. 3 is an exemplary timing diagram of tag interrogation time slots;

FIG. 4 is an exemplary timing diagram of tag interrogation time slotsand

FIG. 5 is an exemplary timing diagram showing timeslot delay betweenslot 1 and slot 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The following description provides a method implemented in an RFIDreader system operating with active transmitter tags which allows theinterrogator antennas to be closely mounted in-line with minimaladjacent separation, while providing high accuracy and speedperformance. This is accomplished by precise overlapping of timedivision multiplexing (TDM) slots for the active tag, with all readerssynchronized with a common synchronization pulse.

The disclosed method allows the effective number of TDM slots to bedoubled, with virtually no increase the overall transaction time, andhas been developed and implemented in live, high-speed, open-roadtolling applications. This approach is particularly applicable inhigh-speed applications requiring antennas to be mounted in-line withadjacent separations of less than twelve feet and also allows forimplementation of a host system that can use “Seen Count” reports fromthe RFID reader to aid in determining vehicle lane position.

With reference to FIG. 1, the system is deployed in a toll plaza byconfiguring each reader for one of the eight active tag slots, insequence across the plaza. In this example there is a reader 10 for lane1 stripe, a second reader 20 for lane 1, a third reader 30 for lane 2stripe, a fourth reader 40 for lane 2, a fifth reader 50 for lane 3stripe, a sixth reader 60 for lane 3, seventh reader 70 for lane 4stripe and an eighth reader 80 for lane 4. There are also shown avehicle 6 having an active transmitter tag (not shown), and road surface5. For plazas with more than 8 readers (not shown), the ninth reader isconfigured for slot 1, the tenth reader for slot 2, and so on.Consecutive numbering of these slots in the same plaza continues acrossthe center median for readers serving lanes in the opposite direction.The arrangement of reader transmission and reception windows based ontime slots is labeled in the Figure. Readers 10 and 50 transmit andreceive in TDM Slot 1. Readers 20 and 60 transmit and receive in TDMSlot 2. Readers 30 and 70 transmit and receive in TDM Slot 3 and Readers40 and 80 transmit and receive in TDM Slot 1. As shown, a response 51 toa request 11 transmitted by the reader 10 in the first lane stripe, canbe mistakenly received by the reader 50 in the lane 3 stripe, thuscreating a cross-lane read error.

With the same system as FIG. 1 configured for eight active time slotsinstead of four, as shown in FIG. 2, it is not possible for a reader toreceive an inadvertent response 51 from a tag activated 11 by anotherantenna in the same time slot, because the reader for slot 5 is notactive to receive a signal during TDM slot 1.

FIGS. 3 and 4 illustrate the timing of an Inter-Agency Group (IAG)active tag protocol for two sets of four TDM timeslots, with FIG. 4slightly delayed from FIG. 3. Downlink and uplink slots for each TDMslot are labeled DL and UL, respectively and are always 200 and 800microseconds long, respectively. Note that the IAG slots 1 through 4essentially overlap with slots 5 through 8 and all eight slots requireonly 5.120 milliseconds; only 0.050 milliseconds more time than fourslots without the second group of four IAG slots overlapped.

As shown in FIG. 4, IAG slots 5 through 8 are delayed from IAG slots 1through 4 in FIG. 3 by 50 microseconds. This timing shift eliminates thepossibility of cross-lane reads between antennas separated by up toeight positions, because a signal received by an antenna four positionsaway from the active antenna will not be read because the time slot forthe receiving signal at the antenna four positions away is shifted fromthe time slot of the transmitting antenna.

This method of overlapping two groups of time slots is shown in detailin FIG. 5. Note that the acquire window of Slot 1 lies between the 20microsecond request pulse and the acquire window for Slot 5. This sameinterlaced timing is implemented between Slots 2 and 6, 3 and 7, and 4and 8.

While a system with eight readers and two sets of four interlacedtimeslots is described above, the invention is not limited to thenumbers and timings described in this example. For example a system withfour readers having two sets of two interleaved time periods ispossible, as is a system with six readers having two sets of threeinterleaved time periods. Not all possible time slots need to be used,for example a seven reader system using four TDM slots for the firstfour readers and three additional readers interleaved as described aboveexcept that the fourth TDM slot is unused.

Those skilled in the art will recognize other detailed designs andmethods that can be developed employing the teachings of the presentinvention. The examples provided here are illustrative and do not limitthe scope of the invention, which is defined by the attached claims.

What is claimed is:
 1. An RFID interrogation system comprising: a firstRFID interrogator; a first antenna; and a second antenna, wherein thefirst and second antennas located in proximity to each other and whereinthe first and second antennas are located to direct signals to and toreceive signals from respective first and second interrogation zones,wherein a first downlink signal from the first RFID interrogator istransmitted to the first antenna during a first downlink window, whereina first uplink window is opened and after said first downlink window forreceiving a signal from a first RFID transponder in the firstinterrogation zone; wherein a second downlink signal from the first RFIDinterrogator or from a second RFID interrogator is transmitted to thesecond antenna during a second downlink window, wherein the seconddownlink window occurs after the first downlink window, wherein a seconduplink window is opened after the second downlink window for receiving asignal from a second RFID transponder in the second interrogation zone,wherein the second RFID transponder is different than the first RFIDtransponder, and wherein: said first uplink window comprises a firstacquire window, said second uplink window comprises a second acquirewindow, said first acquire window occurs after said second downlinksignal and said second acquire window occurs after said first acquirewindow, and said first and second acquire windows do not overlap intime.
 2. The system of claim 1, wherein said system is configured toassign a signal reception occurring during said first acquire window asa response to said first interrogation signal.
 3. The system of claim 1,wherein said system is configured to assign a signal reception occurringduring said second acquire window as a response to said secondinterrogation signal.
 4. The system of claim 1 wherein a third antennais located between said first and second antennas and is located todirect third antenna signals to and to receive signals from a thirdinterrogation zone, wherein said third antenna signals do not overlap intime with said first downlink signal or said second downlink signal. 5.The system of claim 4 wherein a fourth antenna is located adjacent tosaid second antenna, but not adjacent to said third antenna and islocated to direct fourth antenna signals to and to receive signals froma fourth interrogation zone, wherein said fourth antenna signals do notoverlap in time with said first downlink signal or said second downlinksignal.
 6. The system of claim 5 wherein a fifth antenna is locatedbetween said second and third antennas and is located to direct fifthantenna signals to and to receive signals from a fifth interrogationzone, wherein said fifth antenna signals do not overlap in time withsaid first downlink signal or said second downlink signal.
 7. The systemof claim 6 wherein a sixth antenna is located adjacent to said fourthantenna, but not adjacent to said second antenna and is located todirect sixth antenna signals to and to receive signals from a sixthinterrogation zone, wherein said sixth antenna signals do not overlap intime with said first downlink signal or said second downlink signal. 8.The system of claim 1, wherein said first and second interrogation zonesare close enough to each other that an RFID transponder can receive bothof said first and second downlink signals.
 9. The system of claim 1,wherein said first and second antennas are separated by additionalantennas and wherein said additional antennas transmit interrogationsignals at different times than said first and second interrogationintervals.